This document relates to circuit assemblies that include integrated circuits (ICs) and interposers.
A typical IC is a small, fragile device with tiny contact pads that cannot be directly connected to large electro-mechanical connectors such as screw connectors or computer-card slots. Therefore, ICs are packaged into sturdier packages having larger contact pads. The packages should preferably be small and allow circuits to be interconnected by short electrical paths to provide high speed and low power consumption. To achieve these objectives, a single package may interconnect multiple circuits. For example, a package may include a number of ICs and discrete circuits attached to a printed circuit board (PCB) to form a sturdy computer card that can be forced into a tight computer-card slot without breakage. The PCB includes interconnect lines that interconnect multiple ICs or other circuits and connect them to a sturdy plug insertable into the slot.
A PCB can be inexpensively manufactured as a laminate of conductive (copper) and insulating layers. While inexpensive, this manufacturing technology does not allow the interconnects and contact pads to have a high density provided by technologies used to manufacture ICs. Therefore, some packages include intermediate substrates between the ICs and the PCBs. An intermediate substrate, called “interposer”, can be formed of silicon or other material to provide denser interconnects. An interposer may also have denser contact pads for attachment to the ICs, and may have larger, farther-spaced contact pads for attachment to the PCB or other interposers. In addition to facilitating IC interconnection, an interposer may absorb some of the thermal expansion stresses resulting from differences in the coefficients of thermal expansion (CTE) between the ICs, the PCB, and other parts of a circuit assembly. (Thermal stresses are a common cause of IC package failure.)
FIG. 1 shows an example package with ICs 110 interconnected by two interposers (“ITP”) 120 and a PCB 130. Each IC 110 may include transistors, resistors, capacitors and/or other circuit elements (not shown) formed in and around a corresponding semiconductor substrate 110S. The ICs can be accessed through their contact pads 110C, which are attached, by solder balls 140, to contact pads 120C.T at the top of interposers 120. Each interposer 120 also includes bottom contact pads 120C.B attached to PCB contact pads 130C by larger solder balls 140′. Each interposer 120 includes interconnect lines 120I (“interconnects”) which provide suitable interconnection between the contact pads 120C.T and 120C.B. Each interposer includes a substrate 120S made of silicon or other material as suitable to absorb thermal stresses and provide desired density of interconnects 120I and top contact pads 120C.T; the bottom contact pads 120C.B are sufficiently large, and are sufficiently far apart, to match the PCB contact pads 130C. At least some of interconnects 120I can be part of redistribution layers (RDLs) 120R.T and 120R.B formed respectively at the top and bottom of the interposer (the redistribution layers include conductive and dielectric layers; the conductive layers provide the contact pads 120C.T and 120C.B). The PCB's interconnects 130I provide interconnection between the PCB contact pads 130C and, for example, a plug 130P connectable to external circuits. An encapsulant (not shown), can be flown over the structure and between the dies, the interposer, and the PCB, and then solidified, to increase the mechanical strength of the assembly and protect it from moisture, alpha particles, and other harmful elements.
As illustrated in FIG. 1, in addition to providing an interface between small IC contact pads 110C and large PCB contact pads 130C, interconnects 120I can also interconnect IC contact pads 110C of the same or different ICs, thus providing an additional interconnect level supplementing the PCB interconnects 130I. Interconnects 120I thus reduce the interconnect load on the PCB and also reduce the lateral size of the package and allow shorter electrical paths between the ICs 110. These advantages could be enhanced if the interposers 120 were merged into a single, larger interposer. However, use of large interposers leads to large stresses on connections 140 and 140′. Therefore, it is desirable to provide more efficient interconnection between different smaller interposers.
One solution to this problem is to use a bridging die 210 (FIGS. 2A, 2B) as described in U.S. pre-grant patent publication 2009/0267238 (Oct. 29, 2009; Joseph et al.). In FIG. 2A, two interposers 120 are attached to the top surface of an intermediate substrate 130 (a ceramic or organic substrate) which has solder balls 140″ on the bottom for attachment to other devices. Dies 110, or stacks of such dies, are each attached to a single one of the interposers. Bridging die 210 overlies, and is attached to, both interposers 120 by solder balls 140 to provide fast and dense interconnects between the interposers.
In FIG. 2B, bridging die 210 underlies the interposers and is glued to the ceramic or organic substrate 130. Die 210 is attached to the interposers by solder balls 140′. See also PCT publication WO 2013/119309 (15 Aug. 2013) and U.S. pre-grant patent publications 2013/0200511 (Aug. 8, 2013, Banijamali) and 2013/0214432 (Aug. 22, 2013; Wu et al.).
If the interposers 120 are thin and fragile, they can be easily damaged during fabrication, and further it is difficult to keep them flat because they easily warp. According to the aforementioned publication US 2009/0267238 by Joseph et al., dies 110 are attached to their respective individual interposers 120, and then the interposers are separately attached to substrate 130. Attaching dies to the interposers is difficult if the interposers are warped and fragile as noted above.
A different fabrication process is described in the aforementioned PCT publication WO 2013/119309 and illustrated in FIG. 3. In that process, before the ICs are attached to the interposers, the interposers are “placed into or otherwise placed in contact with a molding or packaging material” 310. Material 310 extends into a gap 320 between the interposers and appears to hold the interposers together, facilitating the attachment of dies (not shown) to the interposer.
Other circuit assemblies and fabrication methods are desirable to facilitate fabrication and to shorten the interconnects.